Temporary vs. Permanent Denture Base Materials

1. Definition and Purpose

2. Classification of Materials

Temporary Denture Base Materials

1. Shellac base plates - Earliest material; thermoplastic, softened in warm water, poor dimensional stability
2. Wax (baseplate wax) - Used for try-ins, record bases; has no structural strength
3. Self-cured (cold-cure) PMMA - Polymethyl methacrylate activated by chemical initiators; quick setting, easy to use
4. Bis-acryl composite resins - Used for interim crowns/bridges and temporary denture bases; added aesthetic quality
5. Thermoplastic sheets (Triad VLC) - Visible-light-cured urethane dimethacrylate resin; used for custom trays and record bases
6. Autopolymerizing resins - Sprinkle-on or dough technique; simple chair-side use

Permanent Denture Base Materials

1. Heat-cured PMMA (polymethyl methacrylate) - The gold standard; processed by compression molding or injection molding in a water bath or dry heat
2. Milled (CAD/CAM) PMMA discs - Industrially pre-polymerized; high density, low porosity, best dimensional accuracy
3. Metal alloys (cobalt-chromium, titanium) - Used for partial denture frameworks; extremely strong and thin
4. Injection-molded thermoplastic nylon (Valplast) - Flexible partial dentures; metal-free, aesthetic clasps
5. Thermoplastic acetal resin - Rigid, high fracture resistance; used for clasps and frameworks
6. 3D-printed resins (with ISO-validated post-cure) - Emerging digital workflow; definitive use only with fully validated resins
7. Fiber-reinforced PMMA - Glass or polyethylene fibers added to improve impact/flexural strength

3. Key Property Differences

4. Polymerization Methods

Temporary

• Chemical activation (autopolymerization): Benzoyl peroxide initiator + amine activator at room temperature; fast but generates more residual monomer and heat
• Visible-light curing (VLC): Camphorquinone photoinitiator; controlled cure for record bases and trays

Permanent

• Heat curing (water bath, 70°C x 90 min then 100°C x 30 min): Most widely used; produces lowest residual monomer; conventional compression mold or injection mold technique
• Microwave curing: Faster alternative to water bath; acceptable properties when done with proper protocol
• CAD/CAM milling: Industrial polymerization in factory-controlled conditions; highest quality outcome
• 3D printing: Digital fabrication, but requires thorough post-cure to reduce cytotoxic residual monomers

5. Residual Monomer - The Critical Difference

• Cold-cure (temporary): Residual monomer can reach 3-7% - causes mucosal irritation, contact stomatitis, and is a potential allergen
• Heat-cure (permanent): Residual monomer reduced to approximately 0.5% due to higher conversion of monomers
• Milled PMMA: Near-zero residual monomer - most biocompatible option

6. Clinical Applications Summary

7. Ideal Requirements (and How Each Falls Short)

• High transverse strength - permanent materials meet this; temporary do not
• High impact resistance - permanent (esp. fiber-reinforced) perform better
• Dimensional stability - permanent (heat-cure, milled) are superior
• Low residual monomer - permanent materials win clearly
• Good tissue compatibility - permanent materials are safer long-term
• Ease of processing - temporary materials are easier/faster chair-side
• Repairability - both PMMA-based materials are repairable; metals and nylon are not easily repaired
• Aesthetics - both can match gingival color, but permanent materials maintain color longer

Summary

Fabrication Techniques for Dentures

I. COMPRESSION MOLDING TECHNIQUE (Conventional / Compression-Pack Method)

Steps of Compression Molding

Step 1 - Trial Denture (Wax-Up) Approval

Step 2 - Flasking

• The working cast with the wax denture is invested in a dental flask (a two-part metal/chrome-plated flask).
• The lower half (cope) of the flask is filled with dental stone/plaster; the cast is seated to the land area.
• After the lower half sets, a separating medium (e.g., sodium alginate) is applied.
• The upper half (drag) is assembled and filled with dental stone, completely investing the wax denture but leaving the teeth accessible. Some techniques invest teeth in the lower half instead.
• The flask is closed and placed in a flask press to ensure proper closure.

Step 3 - Wax Elimination (Dewaxing)

• The closed flask is placed in boiling water for 5-7 minutes to soften the wax.
• The flask is then opened; all wax is flushed out with boiling water until the mold is completely clean.
• This leaves a negative mold space between the two halves of the stone/plaster, with the artificial teeth held in position in one half.

Step 4 - Separating Medium Application

• A thin film of tin-foil substitute (sodium alginate solution) or cold mold seal is painted onto all plaster/stone surfaces to prevent the acrylic from bonding to the investment material.
• The artificial teeth and the mold surfaces are carefully coated.

Step 5 - Mixing the Resin (PMMA Dough Preparation)

• Polymer (powder) and monomer (liquid) are measured in the manufacturer's recommended ratio (typically 3:1 by volume or 2.5:1 by weight).
• Mixed in a sealed jar or covered container to reduce monomer evaporation.
• The mixture passes through stages: sandy → stringy → dough → rubbery → stiff/dry. Packing is done at the dough stage (when the mix no longer sticks to gloved fingers and forms a smooth, pliable mass).

Step 6 - Packing (Loading the Mold)

• The dough is placed in the mold cavity (usually the lower half, over the denture teeth).
• A polyethylene sheet (separating foil) is placed over the dough.
• The two flask halves are trial-packed: the flask is pressed in a bench press, opened, excess flash is trimmed, and the foil is removed.
• This is repeated (multiple trial closures) until there is minimal to no excess flash.
• The flask is then finally closed (without foil) and clamped in the press.

• Over-packing - excess resin → teeth displaced, vertical dimension increased
• Under-packing - insufficient resin → porosity, voids, weak base

Step 7 - Polymerization (Curing)

Step 8 - Cooling (Bench Cooling / Controlled Cooling)

• After curing, the flask is allowed to cool slowly at room temperature (bench cooling, ~30-60 min) before deflasking.
• Rapid cooling in cold water causes residual stresses and warpage due to differential thermal contraction.

Step 9 - Deflasking

• The flask is opened; stone/plaster is carefully broken away using chisels.
• The denture is retrieved and cleaned of all plaster remnants.

Step 10 - Finishing and Polishing

• Excess acrylic (flash) is trimmed with acrylic burs, stones, and scrapers.
• Occlusion is checked and adjusted on the articulator.
• The denture is progressively polished: pumice slurry → fine pumice → tripoli → chalk (whiting) on a rag wheel to achieve a high gloss finish.

Advantages of Compression Molding

• Simple, widely understood technique
• Low cost - no specialized equipment beyond a flask and water bath
• Reliable and time-tested
• Easy to repair and reline

Disadvantages of Compression Molding

• Dimensional inaccuracy - polymerization shrinkage is not compensated; the closed flask prevents material from flowing to compensate shrinkage
• Incisal pin opening - as resin shrinks, the occlusal vertical dimension increases slightly (0.1-0.5 mm), requiring post-processing occlusal adjustment
• Porosity - if trial closures are inadequate or curing is rushed
• Flash and thick borders - if over-packed
• Technique-sensitive - many variables affect the outcome

II. OTHER DENTURE FABRICATION TECHNIQUES

1. Injection Molding Technique

Principle

Key Features

• The flask is completely sealed before resin introduction - no flash, no opening
• As polymerization shrinkage occurs, more resin is continuously injected to compensate - this is the fundamental advantage
• Systems include: Ivocap (Ivoclar), SR-Ivocap, and Deflex injection systems

Steps

1. Flasking and dewaxing are performed as in compression molding
2. The flask halves are clamped shut permanently
3. A syringe/injection system forces resin into the mold through sprues under pressure (typically 5-6 bar)
4. Heat curing proceeds in the same water bath cycle
5. The pressure reservoir continues to supply resin as shrinkage occurs

Advantages over Compression Molding

• Better dimensional accuracy and less vertical dimension change
• Lower polymerization shrinkage compensation
• Reduced porosity
• Better adaptation in posterior palatal seal and border areas
• Less post-processing occlusal adjustment needed

Disadvantages

• More expensive equipment required
• More complex technique; requires training
• Sprue channel removal and repair needed

2. Microwave Curing Technique

Principle

Requirements

• Special microwave-compatible flasks (glass or kevlar/nylon - no metal)
• Dedicated dental microwave unit or modified domestic microwave

Curing Cycle (typical)

• 500 W × 3 min → rest 3 min → 500 W × 3 min (various protocols exist per manufacturer)
• Alternatively: 90 W × 14 min (low-slow) for better properties

Advantages

• Very fast processing (minutes vs. hours)
• Clinically acceptable properties
• Convenient for urgent cases and repairs

Disadvantages

• Special flasks required (added cost)
• Risk of porosity if power is too high (overheating)
• Residual monomer may be slightly higher than optimal water-bath curing
• Not as dimensionally accurate as long-cycle water-bath curing

3. CAD/CAM Milling (Subtractive Manufacturing)

Principle

Workflow

1. Digital impressions (intraoral scanner) or scanning of conventional impressions/casts
2. Virtual articulation and tooth setup in CAD software (e.g., Exocad, 3Shape)
3. CAM milling - a 4- or 5-axis milling machine cuts the denture base from an industrially polymerized PMMA disc
4. Teeth bonded or milled separately and bonded to the base

Advantages

• Highest dimensional accuracy of all techniques
• Near-zero residual monomer (industrial polymerization = >99.5% conversion)
• Lowest porosity
• Digital record storage - exact duplicate can be remilled anytime
• Consistent, reproducible results
• No flask, no wax elimination, no packing

Disadvantages

• High equipment cost (scanner + milling unit + software)
• High material cost (pre-polymerized discs)
• Material waste (subtractive process)
• Tooth-to-base bonding interface can be a weak point if not properly managed

4. 3D Printing / Additive Manufacturing

Principle

Workflow

1. Digital design as in CAD/CAM
2. STL file sent to the printer
3. Layer-by-layer photopolymerization builds the denture base
4. Post-curing in a UV light chamber is mandatory to reduce cytotoxic residual monomers and reach adequate mechanical properties

Advantages

• Very fast fabrication
• Minimal material waste (additive)
• Complex geometries achievable
• Ideal for try-in bases and interim dentures
• Digital archive for easy reproduction

Disadvantages

• Lower flexural and impact strength than heat-cured or milled PMMA
• Higher residual monomer if post-cure is inadequate - biocompatibility concern
• Post-cure protocol is critical and technique-sensitive
• Currently used primarily for interim/trial dentures; definitive use requires ISO-validated resins with full post-cure protocols
• Surface quality may require additional finishing

5. Visible Light Curing (VLC) Technique

Principle

Examples

• Triad TruTray / Triad VLC (Dentsply)
• Reline It / Eclipse (Dentsply)

Advantages

• Quick, chair-side or easy lab use
• Lower polymerization shrinkage than cold-cure PMMA
• Low residual monomer

Disadvantages

• Light penetration limited to ~6-8 mm - inadequate for thick denture bases
• Mechanical properties inferior to heat-cured PMMA
• Not suitable as definitive complete denture base

6. Cold-Cure (Autopolymerizing / Self-Cure) Technique

• Discussed in detail as a temporary material in the previous session
• Used for repairs, relines, immediate dentures, and record bases
• Chemical activation (benzoyl peroxide + amine) at room temperature
• Fast, chair-side; however, has the highest residual monomer content and greatest porosity of all methods

III. Comparative Summary Table

IV. Clinical Decision-Making

• Budget patient, conventional workflow - Compression molding remains the practical gold standard
• High-precision cases (bruxists, neuromuscular cases) - Injection molding or CAD/CAM milling
• Urgent or emergency dentures - Microwave curing or cold-cure
• Digital practice - CAD/CAM or 3D printing workflow
• Record bases and custom trays - VLC or cold-cure